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Quantum devices will revolutionise computing, offering up a whole new way of operating that will allow huge calculations to be completed that classical computers simply can't do. We're now one step closer to quantum computing becoming a reality thanks to new research led by a team of University of Sydney physicists, who have found a new way to detect changes in charges smaller than one electron.

The research, published in this week's edition of top physics journal Physical Review Letters, brings scaling-up quantum devices to useful sizes closer to reality.

Alice Mahoney and James Colless, School of Physics PhD students, have found a new charge detection method that brings scaling-up quantum devices to useful sizes closer to reality. Here they are preparing a dilution refrigerator for experiments on quantum dots, as temperatures close to absolute zero are required to study the quantum behaviour.

"Our new method for detecting charge in quantum systems is exciting and has implications for a range of nanotechnologies," said Associate Professor David Reilly, from the ARC Centre for Engineered Quantum Systems in the School of Physics at the University of Sydney.

"We've been successful in finding a new, more convenient way to detect changes in charge of a single electron on quantum dots. Quantum dots are nanoscale systems that can confine or trap single electrons," explained Associate Professor Reilly.

"Electrons confined to quantum dots are very nice systems for storing and manipulating quantum information, where data is encoded in the quantum mechanical aspects of the electron. Our goal is to scale-up a large number of quantum dots to ultimately create a machine to process quantum information - a quantum computer."

Ever since Nobel Prize winner Richard Feynman highlighted the potential of quantum computing in the 1980s, scientists have been attempting to build quantum computers capable of solving some of the largest and most complex problems, with much greater efficiency than conventional computers.

"We've focused on quantum dots as their properties can be tuned in the laboratory - unlike atoms that have their properties fixed in nature, we can control the energy spectrum of quantum dots, for instance, by turning a knob in the lab."

"Being able to detect single electron charges on the quantum dots is absolutely essential, as it's the way information is retrieved from such quantum mechanical systems. We call it 'read-out' and it's analogous to reading information from the memory or a hard drive in a regular classical computer," said Associate Professor Reilly.

"Without the ability to read-out quantum information, we have no way of getting the answer to a computation!"

Associate Professor David Reilly, Associate Professor Andrew Doherty, with their PhD students James Colless, Alice Mahoney and John Hornibrook, and two scientists from the University of California, Santa Barbara, have found a new way to detect changes in charge smaller than one electron on quantum dots.

The team, including School of Physics PhD students James Colless, Alice Mahoney and John Hornibrook, and Associate Professor Andrew Doherty and Associate Professor David Reilly, with two scientists from the University of California, Santa Barbara, have found a new way of detecting charge on the quantum dots using the gate electrodes already in the system.

"Previously, sensitive electrometers which measure minute charges were used to read-out the electron state on quantum dots. These work well, but they are somewhat separate devices built onto the ends of the quantum dot system. They are a bit like having microphones nearby that can pick up the sound of electrons," explained Associate Professor Reilly.

"What we have shown is that the gates or electrodes that are already in place to create the quantum dot in the first place, can also act as read-out detectors. This means you don't need separate devices and you don't need to worry about how to place those separate electrometer devices."

"Whereas the old system was like having microphones nearby to detect sound, our new system could be likened to using the walls of a room as in-built microphones - you don't need separate microphones for every room of the house, just use the walls as microphones," said Associate Professor Reilly.

"Our new method makes the whole quantum system easier to build and use, as adding nanoscale electrometers for every quantum dot in a million-dot-array is a hard problem. By using the electrodes already in the system, we've found an efficient new way to measure charge in the big quantum systems of the future."

The new method of detection allows for read-out in large dot arrays with no limitation on the size of the array for the read-out method to work.

James Colless, whose PhD research contributed greatly to the finding, said, "The technologies that we are developing are part of a global research effort to advance the prospect of quantum computing. In a similar way to how billions of transistors can now be placed on a single silicon computer chip, in the future we would like to engineer semiconductor chips containing huge numbers of interacting quantum two-level systems - called qubits. The work presented in this paper suggests a new method of reading out qubits that enables this goal."

"For me this paper is the culmination of much hard work over the past few years. From having started with just a few pieces of electrical equipment and materials, the fact that we are now able to measure signals from individual electrons is quite incredible," said James.

The team have focused on using this technique in the context of scaled-up quantum computing, but recognise that the method is equally applicable to detecting charge motion in a wide range of electronic devices.

Alice Mahoney, who is focusing on this work for her PhD research and is a lead author on the paper, said, "The ability to read-out information from single electrons is a key step towards being able to coherently control quantum systems."

"The use of gate sensors in this system means that we have a means of detecting electrons without having to directly contact them, using 'quantum capacitance'. This is similar to classical capacitance, which is used in, for example, touch screens on iPhones."

The team has already started to use this method as a new research tool to detect charge in new regimes and devices where traditional methods have failed. The new method adds to the tool-kit of techniques for controlling and manipulating the quantum world.

The work is supported by the Australian Research Council (through the Centre of Excellence for Engineered Quantum Systems) and US Government Intelligence Advanced Research Projects Activity (IARPA).